This description relates to liquid crystal displays.
There are several types of liquid crystal displays, such as twisted nematic liquid crystal displays, vertically aligned liquid crystal displays, multiple domain vertically aligned liquid crystal displays, and optically compensated birefringence mode (OCB mode, also referred to as optically compensated bend mode, or π cell) liquid crystal displays. OCB mode liquid crystal displays have fast responses and can show movie or animation having fast changing scenes with high clarity. OCB mode liquid crystal displays are described in U.S. Pat. No. 6,069,620, “Driving Method of Liquid Crystal Display Device” and U.S. Pat. No. 6,005,646, “Voltage Application Driving Method,” the contents of which are incorporated by reference.
An OCB mode liquid crystal display has an array of pixels that can be independently controlled to show different gray-scales. Referring to FIGS. 1A and 1B, each pixel includes a liquid crystal cell 94 that is positioned between an upper substrate 110 and a lower substrate 120. Attached to the upper substrate 110 are color filters and a common electrode (not shown). Attached to the lower substrate 120 are thin film transistors and pixel electrodes (not shown). The common electrode is driven by a common voltage Vcom, and the pixel electrode is driven by a driving signal Vs. A characteristic of the OCB mode liquid crystal display is that the liquid crystal cell 94 changes between a “splay orientation state” and a “bend orientation state” depending on the voltage applied across the liquid crystal cell 94.
Referring to FIG. 1A, when the voltage difference between the common electrode and the pixel electrode is zero (both Vcom and Vs are equal to zero), the liquid crystal molecules 100 are arranged in the splay state. Referring to FIG. 1B, when the common electrode is maintained at ground voltage (e.g., 0V), and an AC driving signal having an amplitude above a threshold voltage (e.g., 2V) is applied to the pixel electrode, an electric field is created to cause the liquid crystal molecules to be oriented in the bend state. After the liquid crystal cell 94 enters the bend state, adjusting the level of the driving voltage (which is still above the threshold voltage) causes the liquid crystal molecules to change orientation, modifying the amount of light that passes through the liquid crystal cell 94, thereby generating gray-scale. If the driving voltage drops below the threshold voltage, the liquid crystal cell 94 returns to the splay state.
FIG. 2 shows a waveform 200 of a conventional driving signal Vs for driving a pixel electrode of the OCB mode liquid crystal display upon start-up of the display. The common electrode is maintained at ground voltage. Initially, the liquid crystal cell 94 are in the splay state (as shown in FIG. 1A). After the driving signal Vs is applied to the pixel electrode, the liquid crystal cell 94 gradually changes from the splay state to the bend state (as shown in FIG. 1B). The driving signal Vs is a 60 Hz square wave that alternates between 10V and −10V (see portion 202 of the waveform 200). The frequency 60 Hz is used because the refresh rate of the display is 60 Hz. After the liquid crystal cell 94 changes to the bend state, the display can start to show images by driving the pixel electrode according image signals (see portion 204 of waveform 200). In one example, in the bend state, when Vs=2V, the pixel shows full white, when Vs=7V, the pixel shows full black, and when Vs is between 2V to 7V, the pixel shows a gray-scale between full white and full black.
FIG. 2 also shows a waveform 206 of a driving signal VL for driving a backlight module of the display. Before the liquid crystal cells 94 change to the bend state, the driving signal VL is low (208) so that the backlight does not turn on. After the liquid crystal cells 94 change to the bend state, the driving signal VL becomes high (210) so that the backlight is turned on, allowing the user to see images formed by the pixels.